1. Field of the Invention
The present invention relates, in general, to diffractive light modulators and, more particularly, to a diffractive light modulator, in which the lower support for mirrors is configured in consideration of the internal intrinsic stress of a mirror, thus improving the flatness of a mirror surface.
2. Description of the Related Art
Generally, an optical signal processing technology has advantages in that a great amount of data can be quickly processed in a parallel manner, unlike a conventional digital information processing technology which cannot process a great amount of data in real time. Studies have been conducted on the design and production of binary phase filters, optical logic gates, light amplifiers, image processing techniques, optical devices, and light modulators using a spatial light modulation theory.
A spatial light modulator is applied to an optical memory, optical display device, printer, optical interconnection and hologram fields, and studies have been conducted to develop a display device employing the spatial light modulator.
An example of conventional spatial light modulators is embodied by a recess type thin-film piezoelectric light modulator, which is shown in the sectional view of FIG. 1 and is developed and proposed by Samsung Electro-Mechanics of Korea.
As shown in FIG. 1, the conventional recess type thin-film piezoelectric light modulator includes a plurality of silicon substrates 101 and a plurality of elements 110.
In the related art, the elements 110 may have a predetermined constant width and may be arranged with a regular pattern to form a recess type thin-film piezoelectric light modulator. Alternatively, elements 110 having different widths may be alternately arranged to form a recess type thin-film piezoelectric light modulator. As a further alternative, elements 110 may be arranged at regular intervals which are almost equal to the width of each of the elements 110. In this case, the micromirrors, which are formed on the entire upper surfaces of the silicon substrates 101, diffract the incident light by reflecting the light.
Each of the silicon substrates 101 has a recess thereon to provide an air space to each of the elements 110, with an insulating layer 102 vapor-deposited on the upper surface of the silicon substrate 101. Both sides of each of the elements 110 are attached to both sides of the silicon substrate 101 outside the recess.
Each of the elements 110 has a bar shape, and the lower surfaces of both sides thereof are attached to both sides of the silicon substrate 101 outside the recess of the substrate 101. Thus, the intermediate portion of the element 110 is spaced apart from the recess of the silicon substrate 101. The element 110 includes a lower support 111, the portion of which placed above the recess of the silicon substrate 101 is movable in a vertical direction.
Furthermore, the element 110 includes a lower electrode layer 112, which is formed on the left side of the lower support 111 and provides a piezoelectric voltage. A piezoelectric material layer 113 is formed on the lower electrode layer 112 and, when a voltage is applied to both sides of the layer 113, a vertical actuating force is generated in the layer 113 through contraction and expansion thereof. The element 110 further includes an upper electrode layer 114, which is formed on the piezoelectric material layer 113 and applies a piezoelectric voltage to the piezoelectric material layer 113.
The element 110 further includes a lower electrode layer 112′, which is formed on the right side of the lower support 111 and provides a piezoelectric voltage. A piezoelectric material layer 113′ is formed on the lower electrode layer 112′ and, when a voltage is applied to both sides of the layer 113′, a vertical actuating force is generated in the layer 113′ through contraction and expansion thereof. The element 110 further includes an upper electrode layer 114′, which is formed on the piezoelectric material layer 113′ and applies the piezoelectric voltage to the piezoelectric material layer 113′.
An upper micromirror 115 is provided on the intermediate portion of the lower support 111 and reflects incident light which has been received by a metal layer.
U.S. patent application Ser. No. 10/952,556, still pending discloses a raised type light modulator, which is different from the above-mentioned recess type light modulator.
Meanwhile, the light modulators described in the patents of Samsung Electro-Mechanics, etc. can be used as devices to display images. In this case, a minimum of two adjacent elements may form a single pixel. Of course, three elements may form a single pixel, or four or six elements may form a single pixel.
Furthermore, U.S. patent application Ser. No. 10/952,573 now U.S. Pat. No. 7,173,751 B2 entitled “an open hole type diffractive light modulator” provides an open hole-based diffractive light modulator, which has a lower micromirror on a silicon substrate, with open holes formed in a lower support so that upper and lower micromirrors can form pixels. In other words, a plurality of open holes is formed in the lower support of FIG. 1.
The open hole type diffractive light modulator must use mirrors for light treatment, thus requiring high flatness of mirror surface to provide desired operational performance thereof. However, because the open holes of this type light modulator are formed in the lower support, bending is undesirably caused in the lower support.
FIG. 2A is a plan view of part of an embodiment of conventional open hole type diffractive light modulators, in which a plurality of open holes is formed on a lower support in directions perpendicular to the longitudinal axis of the lower support. FIG. 2B is a plan view of part of another embodiment of conventional open hole type diffractive light modulators, in which a plurality of open holes is formed on a lower support in directions parallel to the longitudinal axis of the lower support.
A sectional surface of each of the conventional open hole type diffractive light modulators taken along line XA-XA′ of FIG. 2A is illustrated in FIG. 3A. As shown in FIG. 3A, the intermediate portion of the open hole type diffractive light modulator slightly sags to be simply bent downwards. Because the open hole type diffractive light modulator executes a light modulation using the intermediate portion, effect imposed on the light modulation by such simple bending of the light modulator along the line XA-XA′ is negligible. (In the case of a sectional view taken along line XB-XB′ of FIG. 2B, the effect is also negligible in the same manner)
FIG. 3B illustrates a sectional surface of the open hole type diffractive light modulator taken along line YA-YA′ of FIG. 2A. As shown in FIG. 3B, the intermediate portion of the light modulator is complicatedly bent downwards at that intermediate portion. If the wavelength of incident light is set to λ the phase difference typically required by the diffractive light modulator is λ/4, so that the complicated bending along line YA-YA′ of FIG. 2A remarkably reduces optical efficiency of the light modulator, unlike the simple bending of the light modulator along the line XA-XA′.
The bending along line YA-YA′ of FIG. 2A occurs when the upper micromirror 115 and the lower support 111 having different stresses are attached to each other as shown in FIG. 4. In other words, the upper micromirror 115 is a metal layer having an intrinsic stress, while the lower support 111 is a nitride layer having an intrinsic stress. The intrinsic stress of the upper micromirror 115 is different from the intrinsic stress of the lower support 111. Thus, as illustrated in FIG. 4, if the upper micromirror 115 and the lower support 111 having different stresses are attached to each other, bending occurs in the mirror surface.